This invention relates generally to the condensation of vapor and the vaporization of liquid by indirect heat exchange and is particularly applicable for use with a cryogenic air separation system.
An important aspect of a cryogenic air separation system employing a double column is the condensation of the higher pressure column top vapor against boiling lower pressure column bottom liquid to provide reflux for the columns and boilup for the lower pressure column. This heat exchange is generally carried out in a shell and tube heat exchanger or a brazed aluminum heat exchanger. Since the bottom liquid comprises oxygen, it typically is processed within the tubes of the heat exchanger while the higher pressure column top vapor is processed on the shell side of the heat exchanger.
The heat exchangers for this purpose are of two main types. In the thermosyphon configuration the oxygen liquid enters the tubes at the bottom and is vaporized as it passes up the tubes. In the downflow configuration the oxygen liquid is vaporized as it flows downwardly within the tubes. While both of these configurations ensure safe operation of the heat exchanger, which is an important consideration when liquid oxygen is being vaporized, both of these configurations have disadvantages. The thermosyphon configuration suffers from the disadvantage of requiring a larger temperature difference between the hotter and the colder fluids due to the hydrostatic head at the inlet to the boiling section. The downflow configuration requires special flow distributors for effective liquid distribution to the tube inlets, significantly increasing the cost and complexity of the system.
Accordingly, it is an object of this invention to provide an improved condensation and vaporization system which can be effectively employed to vaporize liquid oxygen such as in carrying out cryogenic air separation.
The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention, one aspect of which is:
A method for carrying out cryogenic air separation comprising:
(A) separating feed air within a higher pressure column by cryogenic rectification to produce nitrogen-enriched vapor and oxygen-enriched fluid, passing oxygen-enriched fluid from the higher pressure column into a lower pressure column, and producing by cryogenic rectification oxygen-rich liquid within the lower pressure column;
(B) passing nitrogen-enriched vapor into a plurality of vertically oriented tubes, said tubes each having a fluted internal surface and an outer surface having a plurality of cavities;
(C) passing nitrogen-enriched vapor downwardly within the tubes, passing oxygen-rich liquid downwardly along the outer surfaces of the tubes, and condensing nitrogen-enriched vapor by indirect heat exchange with partially vaporizing oxygen-rich liquid to produce oxygen-rich vapor and remaining oxygen-rich liquid; and
(D) recovering at least some of at least one of the oxygen-rich vapor and remaining oxygen-rich liquid as product oxygen.
Another aspect of the invention is:
A method for boiling and condensing fluids comprising:
(A) providing a condenser/boiler having a plurality of vertically oriented tubes, each of said tubes having a top entrance and bottom exit and having a fluted internal surface and an outer surface having a plurality of cavities;
(B) passing vapor into the entrances of the tubes and downwardly within the tubes, condensing the vapor within the tubes, and withdrawing the resulting condensate from the bottom exits of the tubes;
(C) passing liquid downwardly along the outer surfaces of the tubes and partially vaporizing the downflowing liquid to produce vaporized liquid and remaining liquid; and
(D) collecting remaining liquid and recirculating remaining liquid to the outer surfaces of the tubes for downflow thereon.
Yet another aspect of this invention is:
Apparatus for boiling and condensing fluids comprising:
(A) a plurality of longitudinally oriented tubes, each of said tubes having an entrance and an exit and having a fluted internal surface and an outer surface having a plurality of cavities;
(B) means for passing vapor into the entrances of the tubes and means for withdrawing liquid from the exits of the tubes;
(C) means for providing liquid to the outer surfaces of the tubes for downflow thereon; and
(D) means for collecting liquid from the outer surfaces of the tubes and recirculating said collected liquid to the outer surfaces of the tubes.
As used herein, the term xe2x80x9cfeed airxe2x80x9d means a mixture comprising primarily oxygen and nitrogen, such as ambient air.
As used herein, the term xe2x80x9cfluted surfacexe2x80x9d means a surface having longitudinally running raised ribs. An example of a fluted surface is illustrated in FIG. 4.
As used herein the term xe2x80x9ccolumnxe2x80x9d means a distillation or fractionation column or zone, i.e. a contacting column or zone, wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, as for example, by contacting of the vapor and liquid phases on a series of vertically spaced trays or plates mounted within the column and/or on packing elements such as structured or random packing. For a further discussion of distillation columns, see the Chemical Engineer""s Handbook, fifth edition, edited by R. H. Perry and C. H. Chilton, McGraw-Hill Book Company, New York, Section 13, The Continuous Distillation Process. 
The term-xe2x80x9cdouble columnxe2x80x9d is used to mean a higher pressure column having its upper portion in heat exchange relation with the lower portion of a lower pressure column. A further discussion of double columns appears in Ruheman xe2x80x9cThe Separation of Gasesxe2x80x9d, Oxford University Press, 1949, Chapter VII, Commercial Air Separation.
Vapor and liquid contacting separation processes depend on the difference in vapor pressures for the components. The high vapor pressure (or more volatile or low boiling) component will tend to concentrate in the vapor phase whereas the low vapor pressure (or less volatile or high boiling) component will tend to concentrate in the liquid phase. Distillation is the separation process whereby heating of a liquid mixture can be used to concentrate the more volatile component(s) in the vapor phase and thereby the less volatile component(s) in the liquid phase. Partial condensation is the separation process whereby cooling of a vapor mixture can be used to concentrate the volatile component(s) in the vapor phase and thereby the less volatile component(s) in the liquid phase. Rectification, or continuous distillation, is the separation process that combines successive partial vaporizations and condensations as obtained by a countercurrent treatment of the vapor and liquid phases. The countercurrent contacting of the vapor and liquid phases can be adiabatic or nonadiabatic and can include integral (stagewise) or differential (continuous) contact between the phases. Separation process arrangements that utilize the principles of rectification to separate mixtures are often interchangeably termed rectification columns, distillation columns, or fractionation columns. Cryogenic rectification is a rectification process carried out at least in part at temperatures at or below 150 degrees Kelvin (K).
As used herein the term xe2x80x9cindirect heat exchangexe2x80x9d means the bringing of two fluids into heat exchange relation without any physical contact or intermixing of the fluids with each other.